Great insight regarding famous quotes. I have found that “myths” get perpetrated by word-of-mouth or perhaps what we can call the chain-letter effect. In some instances, I just can’t find the original source and I can only say something like “attributed to so-and-so.”
I also agree with checking authenticity. This appears to be a common downfall amongst graduate students who fail to check the original sources. I have “doomed” more than one in their defenses, wasting a lot of time. Seems like, nowadays, if it is not available online, it is not worth the time to check. Librarians are still around to help.